This invention relates to rotary head reproducing apparatus, such as the type normally used in video or digital audio tape recording, and, more particularly, to such apparatus that is provided with automatic track following (ATF) and which prevents possible lock-up in the event that ATF control locks onto the wrong tracks.
It is known that to record a greater amount of information onto a record medium, such as a magnetic tape, the density with which the information is recorded must be increased. As a result, the tracks in which such information is recorded are made quite narrow. Hence, rotary head recording apparatus, such as analog and digital video tape recorders, digital audio tape recorders and digital data recorders operate to record information in slant tracks having reduced track pitch.
For proper reproduction of the information which is recorded in slant tracks on, for example, a magnetic tape, servo control arrangements have been developed to assure that the rotary heads which are used to scan tracks which have been previously recorded are brought into proper alignment with those tracks. For example, in video tape recorders, it had been common to record a longitudinal "control" track CTL with information which was used as a servo reference signal to effect a servo control over the movement of the tape such that the rotary heads were centered on the respective tracks scanned thereby. While the use of such a CTL track servo control arrangement was effective to obtain proper scanning of tracks having a relatively wide pitch, an improved technique was desirable to provide the improved precision needed for accurate track following of tracks having a relatively narrow pitch.
Accordingly, an automatic track following arrangement has been developed in which tracking control signals are recorded in the information tracks themselves (i.e. in the same slant tracks in which the video, audio or data signals are recorded) and these tracking control signals are reproduced by the rotary heads and used to detect and correct tracking errors as those heads scan the tracks. For example, the tracking control signals may take the form of a pilot signal admitting of a particular frequency that is separate and distinct from the frequencies associated with the recorded information. Tracking errors are determined as a function of the signal level of the reproduced pilot signal. In one proposal, the pilot signal is recorded in a particular track pattern in respective areas of each track. For example, a track may be provided with an ATF area at the beginning portion thereof, referred to as the "head entering region" and may be arranged in a particular pattern to facilitate detection thereof.
A rotary head assembly generally is arranged with heads exhibiting respectively different azimuth angles so that when a head scans a track in which signals had been recorded with the same azimuth angle as that head, crosstalk signals that may be picked up from adjacent tracks and which were recorded with different azimuth angles exhibit reduced signal levels because of the phenomenon known as azimuth loss. Nevertheless, pilot signals which are recorded in those adjacent tracks and which are picked up as crosstalk components may be sufficiently detected, and the relative levels of those crosstalk pilot signals are used to determine whether the head is properly centered on the track then being scanned thereby.
In copending U.S. patent application Ser. No. 08/029,583, filed Mar. 11, 1993, and assigned to the assignee of the instant invention, various pilot signal patterns are discussed. For example, pilot signals of two different frequencies are recorded in alternate tracks with the pilot signal of one frequency being used as an identification signal from which a sampling pulse is derived, and the pilot signal of the other frequency being used as a tracking detection signal whose level is sampled by the aforementioned sampling pulse. Tracking errors are sensed by comparing the relative levels of the sampled tracking detection signals which are picked up as crosstalk components when alternate tracks are scanned.
In another ATF arrangement discussed in the aforementioned copending application, identification signals are recorded in alternate tracks and the tracking detection signal is recorded in only alternate ones of the remaining tracks. In this arrangement, the track pattern repeats itself every four tracks with one-out-of-four of these tracks having neither identification nor detecting signals recorded therein. After these four tracks are scanned, the displacement, or tracking error, of the heads relative to the tracks scanned thereby is detected as a function of the sampled crosstalk component that is picked up when the track containing the identification signal is scanned.
Effective tracking control is obtained from the aforementioned as well as other pilot signal patterns that are recorded in ATF areas located in the vicinities of the beginning and ending portions of the tracks (commonly referred to, as aforesaid, as the head-entering and head-leaving regions of the tracks). Although the identification and tracking detection signals are recorded as distinguishable pilot frequencies, other suitable identifying characteristics can be used to permit the discrimination of identification and detection signals from other information recorded in the tracks and to permit the distinguishing of identification signals from detection signals. For example, the respective signals may exhibit different data patterns, ID portions, or the like.
While the ATF operation which uses the aforementioned pattern of tracking control signals generally provides accurate tracking error correction, it has been found that, if a head of one azimuth angle is in proper alignment with a track that had been recorded by a head of a different azimuth angle, that is, if the heads scan tracks which are one track pitch out-of-phase, the ATF operation may result in "lock-up" of this out-of-phase relationship for a prolonged period of time. For example, such lock-up may remain for up to about 1.5 seconds until the ATF control circuitry recovers the proper tracking condition. If the rotary head apparatus is incorporated into, for example, a video reproducing device, and if that video reproducing device inhibits a video signal from being supplied as an output signal until the proper ATF condition is obtained, this may result in the reproduction of a "blank" video picture which, in turn, may lead a user of the device to conclude that the device has failed.